1. Field of the Invention
This invention relates generally to devices for detecting liquids and more particularly to a detection device for organic liquids that has an electrically resistive sensor element which is sensitive to exposure to organic liquids.
2. Description of the Prior Art
The detection of organic liquids and particularly hydrocarbons such as fuels, solvents and reagents has long been commercially important. Many of these organic liquids are hazardous and knowledge of their presence can be useful in protecting property and the environment. In recent years the problem of hazardous organic liquids leaking from underground tanks and their associated piping has received a great deal of attention in both the public and private sectors. The ability to rapidly detect a leak of a variety of organic liquids from an underground source, with a minimum of ambiguity, is of great commercial and social importance.
Detectors have long been available which can detect organic vapors and gases and thereby imply the possible presence of an organic liquid. Catalytic combustion detectors, metal oxide semiconductor and flame combustion detectors are all examples of devices which can sense the presence of organic vapors and/or gases. However, these detectors cannot discern whether the source of the vapor or gas is local or remote, liquid or gaseous, a change in the "normal" background, the artifact of an old leak, or naturally occurring materials such as methane. Thus while alerting the user to the presence of an organic gas or vapor, they can be ambiguous as to its source. Also, some hydrocarbons such as motor oil produce little or no vapor and thus may fall below the threshold of vapor/gas sensors.
Organic liquids, such as motor fuels, are sometimes held in tanks constructed of fiberglass-reinforced plastic which use "double wall" construction. A desirable place to monitor such tanks for leakage is in the space between the inner and outer wall; the so-called "annulus". These tanks often outgas organic products used in their construction. The outgassed products may be detected by hydrocarbon vapor/gas sensors and so provide a rich and variable signal background which can lead to false alarms or the setting of detection thresholds so high that true leaks are not noted or are confused with false alarms.
Thermal conductivity devices can detect the presence of a liquid and, using appropriate techniques, can discriminate between organic liquids and other liquids. These devices can require a significant volume of the organic liquid be present, and are subject to error if partially in contact with another heat sink since they measure heat loss.
Float devices can also detect organic liquids, but must be combined with other devices to rule out other liquids such as water. Further, float devices must be installed in a way that takes gravity into account. Additionally, these devices may not discriminate between water and hydrocarbon/water mixtures such as acetone/water. They are relatively bulky and require a significant volume of liquid to alarm. Further, they often rely on the organic liquid floating on top of any water present. This would not be the case with products such as many halocarbon solvents.
Charles Ford in U.S. Pat. No. 2,691,134 and Lee Donaghey in U.S. Pat. No. 4,631,952 teach that rubbers, plastics and other swellable, electrically-insulating materials which have been loaded with particles of an electrically conductive material can be used to sense the presence of hydrocarbon liquids, vapors and gases. James Dolan et al. in U.S. Pat. No. 3,045,198 teach that conductive particles embedded onto the surface of a resilient, electrically-insulating substrate, such as a plastic or rubber, can be used to detect the presence of a hydrocarbon liquid, vapor and gas. However, these detectors fail to discriminate between liquids and high levels of vapors and gases.
The prior art detectors that are activated by both liquid and vapors or gas create uncertainty as to the source of the detected hydrocarbon, and the thermal conductivity and float devices require a significant volume of organic liquid before alarming. It would be very desirable to have a detector which could sense an organic liquid in very small amounts while not detecting vapors or gases. It would be desirable if this detector could operate at ambient temperature and not contain moving parts. Furthermore, it would be desirable to have a detector whose operation did not require electronic circuits which compensated for "background" levels of hydrocarbon gases and vapors.